<!DOCTYPE html>
<html>
    <head>
        <style>
            body,html{
                margin: 0;
                width: 100%;
                height: 100%;
            }
            .left{
                float: left;
                background-color: #cccccc;
                height: 100%;
                width: 30%;
            }
            .right{
                float: left;
                background-color: red;
                height: 100%;
                width: 70%;
            }
        </style>
    </head>
    <body>
        <div class="left"></div>
        <div class="right"></div>
    </body>
</html>